This invention relates to new photoconductive coating compositions, and more particularly to such compositions comprising polymeric binders derived from bromine-containing monomers and to the improved photoconductive coatings prepared from such compositions.
In the known electrophotographic processes, a solid substrate which has been coated with a photoconductive surface is electrostatically charged in the absence of light. The charged surface is then exposed to a light source, such as an image reflected from a printed surface or transmitted through a transparency, which serves to discharge the exposed area while having no effect upon the charge on the unexposed areas which is thus retained in the form of an electrostatic image. The surface areas are then contacted with an electrostatically charged marking powder or dispersion which clings to the charged areas but does not adhere to the uncharged, i.e. exposed, areas. The visible image which is thus formed may then be transferred to another surface resulting in a positive or negative print, or, if desired, it may be fused in order to fix the image directly to the solid substrate itself.
It is evident that the proper selection of a photoconductive system is essential for a successful electrophotographic operation. Among the properties that must be exhibited by such photoconductive materials are included: the ability to accept an electrostatic charge; the ability to hold the charge for a period of time in the absence of light without exhibiting a rapid rate of charge decay; the ability to provide the fixed or developed sheets with excellent print density; the ability to provide an efficient dispersing medium for the pigment particles; inertness of the binder toward the pigment which is being utilized; and, the ability to be readily coated on a solid substrate.
A frequently utilized photoconductive system comprises the combination of a zinc oxide pigment and polyvinyl acetate or a styrene interpolymer binder. These systems, however, do not exhibit the aforementioned properties to an optimum level.
It is recognized in the art that the properties of an electrophotoconductive system are realized not only from the particular photoconductive pigment which is utilized; rather, it is the combination of the pigment with a particular binding agent which is the critical factor. It can be understood, therefore, that the choice of a binding agent in connection with a pigment, such as zinc oxide, is of prime importance in achieving the desired characteristics of the system.
We have now found that photoconductive coatings comprising a photoconductive pigment, such as zinc oxide, bonded with a binding agent comprised of an interpolymer derived from at least one bromine-containing monomer realize the above described requirements for achieving an effective electrophotographic system having desirable characteristics. A particular feature of the present electrophotoconductive compositions is their ability to discharge their electrostatic charge almost instantaneously in the presence of light. Thus, we have found that solid substrates which have been coated with the latter material will accept and hold an electrostatic charge in the absence of light, and upon being exposed to light will immediately release the charge from the exposed areas but will retain the charge on the unexposed areas with a high degree of efficiency and for a period of time sufficient to permit visible development of the electrostatic image formed by the unexposed areas. Furthermore, these photoconductive systems are readily coated onto a variety of solid substrates and provide the resulting printed sheet with excellent density, i.e. with excellent definition of the visible image.